Abstract: Valproic acid (VPA) is a fatty acid antiepileptic with demonstrated antimanic properties, but the molecular mechanism or mechanisms underlying its therapeutic efficacy remain to be elucidated. In view of the increasing evidence demonstrating effects of the first-line antimanic drug, lithium, on protein kinase C (PKC), we investigated the effects of VPA on various aspects of this enzyme. Chronic exposure (6–7 days) of rat C6 glioma cells to “therapeutic” concentrations (0.6 mM) of VPA resulted in decreased PKC activity in both membrane and cytosolic fractions and increased the cytosol/membrane ratio of PKC activity. Western blot analysis revealed isozyme-selective decreases in the levels of PKC α and ε (but not δ or ζ) in both the membrane and cytosolic fractions after chronic VPA exposure; VPA added to reaction mixtures did not alter PKC activity or 3H-phorbol ester binding. Together, these data suggest that chronic VPA indirectly lowers the levels of specific isozymes of PKC in C6 cells. Given the pivotal role of PKC in regulating neuronal signal transduction and modulating intracellular cross-talk between neurotransmitter systems, the specific decreases in PKC α and ε may play a role in the antimanic effects of VPA.